The present invention relates to electrical fault detecting devices, and in particular to such devices for use with a mains alternating current electricity supply.
The illegal tampering with electricity supplies to obtain electricity without cost, or at a reduced cost, is a widespread problem, particularly with respect to domestic electricity supplies. Three main methods of obtaining electricity illegally are used.
Firstly, the live or the live and neutral wires of the supply are bridged. This is done by connecting a pair of wires to the incoming supply wires and then bridging to the consumer fuse board, thereby by-passing current to the meter, causing it to under read. Secondly the live and neutral wires may be exchanged at the input to the meter, and the neutral can be switched to earth. Since the meter is driven normally by an inductive coil connected to the live cable only, this method again causes the meter to under read since the meter driving coil is only slightly energized. Thirdly, a device which supplies a small A.C. voltage directly to the meter can be connected, but in the opposite phase, thus causing the meter to wind backwards.
It is an object of the present invention to provide an apparatus for detecting when electricity is being abstracted illegally from the supply.
Thus, in accordance with the present invention, apparatus for detecting incorrect usage of an alternating current electricity supply comprises first signal generating means, disposed at a first location in the supply, for generating a first signal which varies with a property of the electricity supply in the vicinity of the first location, second signal generating means, disposed at a second location in the supply, for generating a second signal which varies with the property of the electricity supply in the vicinity of the second location and monitoring and actuator means adapted to monitor the two signals and to produce a third, actuating signal when the first and second signals are inconsistent with a correct electricity supply.
Preferably, the first and/or second signal generating means comprises a coiled portion of conductive leads forming part of the supply, and the signals comprise magnetic flux signals generated by the passage of current. The monitoring device may comprise a detecting coil adapted and positioned to detect flux produced by one or both of the first and second coiled portions. Preferably, the currents carried by the conductive leads are in antiphase.
In a correctly-used electricity supply, the live and neutral currents are exactly in antiphase, and thus the detecting coil described above would only detect a negligible amount of flux, since the flux induced by the live and neutral coils would cancel each other out. However, when one of the above three methods of illegal abstraction of electricity is used, the amplitude of the live and neutral signals is no longer equal, producing a net field which is detected by the detecting coil. The signal thus induced in the detecting coil is used to actuate the indicating means to indicate that illegal abstraction of electricity has occurred.
In a preferred embodiment, the first and second inductive coils and the detecting coil are mounted on a common core, preferably a ferrite core. Advantageously, signals from the detecting coil are amplified by an amplifier before being used to actuate the indicating means.
Preferably, the apparatus further comprises a field effect transistor, the actuating signal being fed to the gate of the field effect transistor. Advantageously, the gate of the field effect transistor is actuated by means of a capacitor charged by the signal from the detecting coil. The effect of this is that temporary, accidental aberations in the electricity supply do not actuate the indicating means.
The indicating means may, for example, comprise a light source, which is preferably extinguished in response to an actuating signal from the detecting coil, or may comprise an electrically-actuable counter or meter, for example an electronic counter.
In another embodiment, the first and/or second signal generating means may comprise detecting coils located around a respective conductive lead of the electricity supply, the first and second signals comprising the electrical signals induced in the coils by the passage of current through the leads.
Instead of balancing the flux produced by coils of the actual input cable, electrical signals produced as a result of the passage of electrical current are balanced against each other continuously. Any tampering with the supply between the first and second coils causes an imbalance in the electrical signals produced thereby, and this imbalance is used to actuate the indicator means. The apparatus is of particular benefit when one of the coils is situated at the entry point of the electrical cable into a dwelling and the other coil is situated within the meter housing. This is particularly advantageous since most illegal abstraction of electricity involves forming a bridge between the input and output cables of the electricity meter. The apparatus in accordance with the second aspect of the present invention would detect any such interference.